Dual Fiber Stretchers for Dispersion Compensation

ABSTRACT

An optical system having at least two waveguides that are deformable to provide adjustments to dispersion and path length.

REFERENCE TO PRIOR APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/122,513, filed Dec. 15, 2008 the entirety of which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention broadly relates to optical fiber based dispersioncompensation and in particular to an all fiber based dispersioncompensation system used for Optical Coherence Tomography.

BACKGROUND TO THE INVENTION

Fiber optic systems having light propagating in multiple parallel paths,such as interferometers and Wavelength Division Multiplexed (WDM)systems, often suffer from output distortion due to dispersion mismatchbetween paths. Dispersion mismatch arises from differences betweenoptical paths such as refractive index, optical fiber manufacturingtolerance and the use of different components.

One example of a dispersion sensitive interferometer based device is anOptical Coherence Tomography (OCT) system. An OCT system produces threedimensional images of biological tissues. Dispersion imbalance in an OCTdevice reduces system resolution due to distortion of thepoint-spread-function.

Similarly, optical systems employing WDM signals often suffer dispersionmismatch between co-propagating wavelengths in an optical fiber.Dispersion mismatch between WDM signals in an optical fiber distorts thesignal due to optical pulses travelling at different speeds in a fiber.

One solution of the prior art for compensating interferometer armdispersion imbalance in fiber optic systems is to use a common-pathinterferometer where an autocorrelator matches interferometer pathlengths. However, when the autocorrelator solution is embodied in afiber it introduces further disadvantageous fiber-induced dispersionimbalance. Dispersion can partially be compensated by matching fiberlengths with sub-mm accuracy, but this is difficult to achieve inpractice. Another problem with such systems is that the manufacturingtolerance of fibers, typically within 1%, causes dispersion imbalanceeven when fiber lengths are identical.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a systemor method for compensating for dispersion mismatch, or at least providethe public with a useful choice.

In a first broad aspect the invention consists in an optical system,comprising:

an input adapted to couple light to at least two optical paths, a firstoptical path having a first waveguide, the first waveguide having afirst dispersion parameter, a first optical path length and isdeformable to change the first dispersion parameter and the firstoptical path length,

the second optical path having a second waveguide, the second waveguidehaving a second dispersion parameter, a second optical path length andis deformable to change the second dispersion parameter and the secondoptical path length,

an output adapted to receive light from the at least two optical paths

a first deforming device adapted to deform the first waveguide, and

a second deforming device adapted to deform the second waveguide,

wherein the first deforming device and the second deforming device areoperable to adjust the dispersion of light in the first optical path andmaintain the path length of the first optical path relative to thesecond optical path.

Preferably the first optical path is a first arm of a Mach-Zehnderinterferometer and the second optical path is a second arm of theMach-Zehnder interferometer.

Preferably the first arm has an interferometry output for coupling lightfrom the first arm and an interferometry input for coupling light intothe first arm.

Preferably the light coupled out of the first arm by the interferometryoutput is reflected by a sample under test and coupled into the firstarm by the interferometry input.

Preferably the system is an Optical Coherence Tomography apparatus, theapparatus further comprising:

a broadband light source adapted to transmit to the input,

a detector adapted to receive light from the output, and

a delay line located in either the first or the second optical path.

Preferably the system is Wavelength Division Multiplexing apparatus.

Preferably the first waveguide and the second waveguide have unequaldispersion parameters.

Preferably the dispersion parameter is a second order dispersioncoefficient.

Preferably at least the first waveguide or the second waveguide is anoptical fiber.

Preferably at least the first deforming device or the second deformingdevice is an optical fiber stretcher.

Preferably the first waveguide has a first waveguide dispersion modifiercoefficient, the second waveguide has a second waveguide dispersionmodifier coefficient, and a ratio between the first and seconddispersion parameters, multiplied by their respective strain inducedwaveguide dispersion modifier coefficients, are unequal.

In another broad aspect the invention is said to consist in a method ofarranging an optical system, comprising the steps of:

adapting an light receiver to receive light from a light source andcouple the input light to at least two optical paths,

arranging a first deformable waveguide in a first optical path, thefirst waveguide having a first dispersion parameter and a first opticalpath length, the first waveguide deformable to alter the firstdispersion parameter and the first optical path length,

arranging a second deformable waveguide in a second optical path, thesecond waveguide having a second dispersion parameter and a secondoptical path length, the second waveguide deformable to alter the seconddispersion parameter and the second optical path length,

deforming the first and the second waveguides adjust the first andsecond dispersion parameters and maintain the path length of the firstoptical path relative to the second optical path.

Preferably the method further comprises configuring the first opticalpath is to be a first arm of a Mach-Zehnder interferometer andconfiguring the second optical path to be a second arm of theMach-Zehnder interferometer.

Preferably the method further comprises configuring an interferometryoutput in the first arm has for coupling light from the firstinterferometer arm and an interferometry input for coupling light intothe first interferometer arm.

Preferably the method further comprises transmitting light from theinterferometry output to a sample under test, and receiving lightreflected from the sample in the interferometry input.

Preferably at least the first deforming device or the second deformingdevice is an optical fiber stretcher.

In another broad aspect the invention is said to consist in an opticalsystem, comprising:

an optical path having at least at first and second waveguide,

a first waveguide having an input to receive light and an output totransmit light,

a second waveguide having an input to receive light and an output totransmit light, the input of the second waveguide configurable toreceive light from the input of the first waveguide,

a first deforming device adapted to deform the first waveguide, and

a second deforming device adapted to deform the second waveguide,

wherein the first deforming device and the second deforming device areoperable to adjust the dispersion of light in the optical path and theoptical path length.

Preferably the system is Wavelength Division Multiplexing apparatus.

Preferably the first waveguide and the second waveguide have unequaldispersion parameters.

Preferably the dispersion parameter is a second order dispersioncoefficient.

Preferably at least the first waveguide or the second waveguide is anoptical fiber.

Preferably at least the first deforming device or the second deformingdevice is an optical fiber stretcher.

Preferably the first waveguide has a first waveguide dispersion modifiercoefficient, the second waveguide has a second waveguide dispersionmodifier coefficient, and a ratio between the first and seconddispersion parameters, multiplied by a respective strain inducedwaveguide dispersion modifier coefficients, are unequal.

In another broad aspect the invention is said to consist in a method ofarranging an optical system, comprising:

arranging at least a first and second waveguide in series to define anoptical path, the first waveguide having an input to receive light andan output to transmit light, the second waveguide having an input toreceive light and an output to transmit light.

Preferably the method further comprises deforming the first and thesecond waveguides to adjust the dispersion of the light in the opticalpath.

Preferably method further comprises transmitting a broadband lightsource to the input of the optical path.

The invention consists in the foregoing and also envisages instructionsof which the following gives examples.

This invention may also be said broadly to consist in the parts,elements and features referred to or indicated in the specification ofthe application, individually or collectively, and any or allcombinations of any two or more of said parts, elements or features, andwhere specific integers are mentioned herein which have knownequivalents in the art to which this invention relates, such knownequivalents are deemed to be incorporated herein as if individually setforth.

The term ‘comprising’ as used in this specification means ‘consisting atleast in part of’, that is to say when interpreting statements in thisspecification which include that term, the features, prefaced by thatterm in each statement, all need to be present but other features canalso be present.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred forms of the present invention will now be described withreference to the accompanying drawings in which:

FIG. 1 illustrates an example of an all fiber optical systemincorporating fiber stretching devices to compensate dispersion andmaintain group delay.

FIGS. 2 a -e illustrate point spread functions of an image output froman all-fiber optical coherence tomography system.

FIG. 3 illustrates an example of the all fiber optical system of FIG. 1where two fiber stretching devices are located in one interferometerarm.

FIG. 4 illustrates an example of a WDM system having two fiberstretching devices used to compensate dispersion.

FIG. 5 illustrates an optical system with multiple dispersioncompensated paths.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Optical Coherence Tomography (OCT) is a real-time non-invasive opticalimaging technique that can produce high resolution images of biologicaltissues. The technique is based on low coherence white lightinterferometry where image slices within the depth of a sample placed inone interferometer arm are obtained by scanning an optical time delayline in the reference arm of the interferometer.

OCT systems have been implemented in many different interferometerarrangements. Of particular interest is the use of optical fibers andbroadband fiber couplers to construct an OCT interferometer. Fiber-basedOCT systems present several advantages in terms of compactness,flexibility, and easiness of light distribution to the sample,especially for use in in vivo experiments.

However, most known fiber-based OCT systems still rely on some opticalfree-space components to match path lengths, such as an optical delayline. Further, the use of optical fibers has an inherent chromaticdispersion mismatch problem between the two arms of the interferometer.A sample under test will also often introduce dispersion. Chromaticdispersion broadens the point-spread-function (PSF) of an OCT system anddegrades the axial resolution of the output image. Further still,interferometry requires precision construction of the arms to ensureequal optical path lengths.

FIG. 1 shows an OCT system arranged from optical fiber based devicesincorporating a preferred form of the present invention. The OCT systemis based on a Mach-Zehnder interferometer generally shown to have afirst arm, known as a sample arm 1, and a second arm, known as areference arm 2. The sample arm 1 includes a first fiber stretchingdevice 3 and a broadband optical coupler 4. The reference arm 2 includesa second fiber stretching device 5, a third fiber stretching device 6and a polarisation controller 7. The sample arm 1 and the reference arm2 are combined by a broadband optical coupler 8. Light coupled out ofthe interferometer by coupler 8 is detected by a balanced detector 12. Abroadband light source 9 is coupled into each interferometer arm 1, 2via a polarisation controller 10 and another broadband optical coupler11.

During tomography measurements, light from light source 9 is coupledinto each interferometer arm. Light is output from the sample arm 2 viacoupler 4 and directed toward a sample to be tested 13. A portion of thelight transmitted to the sample 13 is reflected and coupled back intothe fiber setup and input to the reference arm 2 by coupler 4. Thedetector 12 measures the light emitted from the interferometer afterlight in the reference arm 1 and sample arm 2 has been recombined at thecoupler 8. The polarisation controller 10 is adjustable to maximise thefringe contrast at the output.

Variable optical delay is required to scan the depth of the sample 13.The third fiber stretcher 6 shown in the reference arm of theinterferometer provides the required variable optical delay. It ispreferable that the variable delay device 6 is provided by an all fiberdevice such as a piezoelectric actuator fiber stretcher.

Operation of the first and second fiber stretchers will now bedescribed. A fiber having length L and second order dispersioncoefficient β₂ undergoes a change in dispersion upon elastic stretching.Lengthening the fiber by ΔL increases the fiber length-integrateddispersion ø₂=β₂L by Cβ₂ΔL, where C is a modifier coefficient thataccounts for the strain-induced optical geometrical changes of the fiberthat lead to a corresponding modification of the dispersion coefficient.Note that C is typically lower than 1 which means that the dispersion ofa stretched fiber becomes lower than that of an unstretched fiber of thesame length.

The preferred embodiment of the invention has different fiber types (Aand B) with different dispersion coefficients (β₂ ^(A) and β₂ ^(B)) ineach fiber stretcher 3, 5. However fibers with similar or the samedispersion coefficients could also be used although the effects are notas dramatic. Similarly, fibers with different C values could also beused to similar effect. If we stretch (or un-stretch) both fibers by thesame extra length ΔL, the optical path length between the two arms ofthe interferometer is left unchanged, but, advantageously, thedifference in integrated dispersion is modified by (C_(A)β₂ ^(A)-C_(B)β₂^(B)) ΔL. Note that equal group velocity is assumed here, although thesystem functions equally well when group velocities are not equal.Continuous stretching allows for a continuous change in relativedispersion between the two arms of the interferometer. Thereforedispersion arising at the output of the interferometer, caused bydispersion mismatch between the interferometer arms, is compensated forby the use of the described system without resorting to the use of nonfiber based components. It is therefore evident that at least either Cor β₂ must be different between each stretcher to provide dispersioncompensation.

A theoretical explanation is as follows as applied to a Mach-Zehnderinterferometer. A Mach-Zehnder interferometer presents an initial pathlength imbalance ΔL=L_(A)-L_(B) and dispersion imbalance ø₂=ø₂ ^(A)-ø₂^(B). The amounts ΔL_(A) (Equation 3) and ΔL_(B) (Equation 4) by whichone has to stretch the two arms of the interferometer to balance boththe group delay (Equation 1) and the dispersion (Equations 2) are suchthat:

$\begin{matrix}{{{\Delta \; L} + {\Delta \; L_{A}}} = {\Delta \; L_{B}}} & (1) \\{{{\Delta\varphi}_{2} + {C_{A}\beta_{2}^{A}\Delta \; L_{A}}} = {C_{B}\beta_{2}^{B}\Delta \; L_{B}}} & (2) \\{{\Delta \; L_{A}} = {{{- \frac{1}{\kappa - 1}}\frac{{\Delta\varphi}_{2}}{C_{B}\beta_{2}^{B}}} + {\frac{1}{\kappa - 1}\Delta \; L}}} & (3) \\{{\Delta \; L_{B}} = {{{- \frac{1}{\kappa - 1}}\frac{{\Delta\varphi}_{2}}{C_{B}\beta_{2}^{B}}} + {\frac{\kappa}{\kappa - 1}\Delta \; L}}} & (4)\end{matrix}$

Where κ=(C_(A)β₂ ^(A))/(C_(B)β₂ ^(B)) is essentially the ratio betweenthe dispersion coefficients of the two fibers. In equations 3 and 4, thefirst term represents the amount of stretching needed to cancel theoriginal dispersion imbalance while keeping the relative group delayconstant (since these terms are the same for ΔL_(A) and ΔL_(B)) and thesecond term represents the amount of stretching required to vary thepath difference by ΔL without changing the dispersion. It isadvantageous to use fibers with dispersion coefficients as different aspossible (i.e. κ not equal to 1, but rather much larger than 1, close tozero, or negative) to maximize the amount of dispersion that can becompensated within the elastic stretching limit of the fibers used.Typically, silica based fibers can be stretched up to 2% of theiroriginal length.

An absolute value of K much larger or much smaller than 1 leads tocontrol of dispersion and optical delay independently between the twofiber stretchers. For example, when κ much larger than 1, most of thedispersion adjustment is obtained by stretching the highly dispersivefiber A by an amount much smaller than ΔL while stretching fiber Bmainly tunes the relative optical delay.

The preferred fiber stretcher is made from a length of fiber wrappedaround a rubber cylinder that is sandwiched between two plates. Theplates are tightened together to squash the rubber and outwardly expandthe cylinder and therefore the wrapped fiber. The length of wrappedfiber in a fiber stretcher used for experimental verification isapproximately 4 m. However, any length of fiber may be used. Compressiveforces on the rubber cylinder cause the outer diameter to grow and applyan even outward force that stretches the wrapped fiber. The tighteningforce may be applied manually, or under automated control. Automatedcontrol advantageously allows for dynamic stabilisation of dispersionmatching and optical path length by way of a feedback signal from theinterferometer output. This is particularly advantageous as dispersionwill often depend on the scan depth of a sample under test.

To verify the system experimentally the fiber stretcher 3 located in thesample arm is constructed with FiberCore SM800 fiber. This fiber has asingle mode cut-off wavelength of 730 nm and is therefore compatiblewith our broadband optical source (a 85 nm wide SLED source centered at845 nm). This particular fiber used has a dispersion coefficientmeasured to be β₂ ^(A)2^(A=38) ps²/km at 845 nm by white-lightinterferometry. The fiber stretcher 5 located in the reference arm 2 isconstructed with Crystal Fiber LMA-5 photonic crystal fiber (PCF). Thisfiber was chosen for having properties similar to that of the SM800fiber in terms of its single-mode guidance, high transparency in ourwavelength range, and similar core diameter (5 μm versus 5.6 μm for theSM800 fiber). This particular fiber has a significantly differentdispersion parameter of β₂ ^(B)=23 ps²/km at 845 nm, again obtained bywhite-light interferometry. Note that either β₂ ^(A), β₂ ^(B), or bothcould be negative dispersion values if desired.

Note that the use of two different fiber types in the setup introduces adispersion imbalance per se. The bulk of that imbalance is easilycompensated by using appropriate lengths of both fiber types in each armof the interferometer. An appropriate length can be coarsely cut sincethe fiber stretchers will fine tune fiber lengths over several cm withsub-mm accuracy.

For initial setting of the fiber length it is advantageous to be able tomeasure the relative delay between different wavelength components ofthe broadband optical source 9. The fiber length is then adjustableaccordingly. Preferably the light source 9 comprises 3 multiplexed SLEDsources with different center wavelengths that can be switched onindependently.

For experimental verification, in the optical system used for OCT, thesample arm has a first 13 m section of SM800 fiber and 4.3 m of PCFfiber. The reference arm has 12.8 m of SM800 fiber and 4.7 m of PCFfiber. An air gap of 15 cm exists between the coupler 4 and the sample.However, this air gap could be considerably less.

Note that using two arms of identical length of the same fiber does notguarantee perfect dispersion balance. This was particularly strikingduring preliminary experiments entirely based on SM800 fibers where ourdepth resolution was as high as 400 μm instead of the expected 5.1 μm.The discrepancy is due to differences in the order of 1% or less in thedispersion of different batches of fiber as well as longitudinalfluctuations along the fiber lengths. Some discrepancy is expected fornon-telecoms grade fibers where a manufacturing tolerance for fibers istypically 1%. The discrepancy clearly stresses the importance of anall-fiber dispersion compensator for fiber-based optical systems.

To test the response of the interferometer system a measurement of thepoint spread function (PSF) is made. The response of the system shown inFIG. 1 is made by replacing the sample 13 with a mirror (not shown).FIG. 2( a) shows the optimized PSF of our OCT system versus the axialdepth in air. The full-width-at-half-maximum (FWHM) of the PSF is 5.6 μmand is therefore close to the theoretical expectation of 5.1 μmcalculated by taking the Fourier transform of the source intensityspectrum of the light source used for acquiring experimental data.

Note that the light source has a non-Gaussian spectrum which partlyexplains the side lobes of the PSF. Side lobes are further encouraged bysome third-order dispersion imbalance and some polarization effects dueto the fibers not maintaining polarization of the input light.

During testing of the system the sample mirror (not shown) was firstmoved by a certain distance in the air portion of the sample arm. Theextra optical delay is compensated by (un)stretching the fiber of thereference arm only, thereby artificially introducing some dispersionimbalance. Accordingly, the broadened PSF is observable as shown inFIGS. 2( b) and (d). FIGS. 2( b) and (d) correspond to a mirrordisplacement of 1 cm towards and 2 cm away from the fiber end,respectively.

The FWHM of the PSF indicated in FIG. 2( b) is 30% broader than theinitial PSF shown in FIG. 2( a). Further, the FWHM of the PSF indicatedin FIG. 2( d) is 46% broader than the initial PSF shown in FIG. 2( a).For these two cases, the dispersion imbalance has then been compensatedby adjusting the two stretchers simultaneously while leaving the samplemirror fixed. FIGS. 2( c) and 2(e) show that the PSF can be recompressedclose to its original width. Therefore the dispersion compensator of thepresent invention compensates for positive and negative amounts ofdispersion corresponding to about 4 cm of air-equivalent fiber length.

Note that, in the current demonstration, our PCF introduces athird-order dispersion imbalance since it has a dispersion slope β₃=0.04ps³/km which is approximately twice as large as that of the SM800 fiber.The additional amount of third-order dispersion introduced by stretchingthe fibers is negligible in comparison to the initial imbalance due tothe difference in the β₃ coefficients of our two fiber types. Therefore,the stretchers only modify the second-order dispersion coefficient ofthe system and demonstrate that compensation of the full amount ofartificially introduced second-order dispersion is achievable.

Any imbalance of third-order dispersion leading to unwanted ripples inthe PSF may be resolved by using two fibers with different β₂coefficients but identical β₃ coefficients. This is possible due to thegreat flexibility in designing a dispersion coefficient in PCF fiberdesign.

Therefore, by using two fiber stretchers made up of different fibertypes an all-fiber variable dispersion compensator in an OCT system hasbeen shown to independently adjust the delay and the dispersion in thetwo arms of the interferometer.

The optical system of the present invention is made entirely of fiberelements and does not require any critical alignment. This makes thesystem advantageously compact and versatile for use in in vivoexperimentation when applied to an OCT system. Additionally, thetechnique could similarly be used to compensate at least part of thesample dispersion in an OCT system.

Use of the optical system of the present invention does not require thetwo stretchers to be placed in different arms of the interferometer.Instead, the stretchers can be placed in sequence in one particular arm.The operation of such a serial sequence of stretchers is entirelyequivalent to that of parallel stretchers as long as the stretcher thatwas displaced from one arm to the other is operated in the reversedirection. That is, if both stretchers must be stretched in a parallelconfiguration to achieve a certain result, then one must be stretchedand the other unstretched to provide the same outcome in the serialconfiguration. FIG. 3 illustrates an arrangement similar to that shownin FIG. 1 where two fiber stretchers are located in a single arm of aMach-Zehnder interferometer.

In addition, the use of a serial sequence of stretchers is notrestricted to interferometer geometries as it can be used to adjust thedispersion and the group delay of a single piece of fiber, for example,in telecommunication system applications such as WDM systems. FIG. 4illustrates a general arrangement of two fiber stretchers that can besimultaneously stretched and unstretched to maintain group delay whileproviding a change in dispersion seen by propagating light.

Dispersion and optical path length compensation using the inventiveconcepts described herein can be freely operable or alternatively,operated once to calibrate a system before being fixed into position toprevent future movement. It is envisaged this invention could be used bymanufacturers who wish to calibrate a system only once in a product thatis to be sold.

It is further envisaged that the inventive scope is not restricted tostretching optical fibers. Instead, any other suitable types ofwaveguide that provides a dispersion change when stretched, or generallydeformed, may be used in place of the optical fiber used in theforegoing examples.

It is further envisaged that more than two other waveguide stretcherscan be used in parallel or serial arrangement. For example, threewaveguide stretchers can be provided to compensate three paralleloptical paths as long as each stretcher has different dispersionparameters. FIG. 5 illustrates an arrangement of three fiber stretchingdevices arranged in parallel. The parallel paths may further include twoor more stretchers in one path, no stretcher in the second path, and asingle stretcher in the third path. Other similar combinations andexamples of stretchers arranged in optical paths will be evident tothose skilled in the art in light of the foregoing.

In a further embodiment of the present invention the dual fibrestretchers are used in a Fourier OCT system. A Fourier OCT systemrequires no delay line to scan. Instead, the detector, which is normallya photodiode, is replaced with an optical spectrum analyser. Scanning ofthe signal depth is performed by Fourier-transforming the measuredspectral output. In such an arrangement, scanning can be performed athigh speeds by real time Fourier analysis.

1. An optical system, comprising: an input adapted to couple light to atleast two optical paths, a first optical path having a first waveguide,said first waveguide having a first dispersion parameter, a firstoptical path length and is deformable to change said first dispersionparameter and said first optical path length, said second optical pathhaving a second waveguide, said second waveguide having a seconddispersion parameter, a second optical path length and is deformable tochange said second dispersion parameter and said second optical pathlength, an output adapted to receive light from said at least twooptical paths a first deforming device adapted to deform said firstwaveguide, and a second deforming device adapted to deform said secondwaveguide, wherein said first deforming device and said second deformingdevice are operable to adjust said dispersion of light in said firstoptical path and maintain said path length of said first optical pathrelative to said second optical path.
 2. A system as claimed in claim 1,wherein said first optical path is a first arm of a Mach-Zehnderinterferometer and said second optical path is a second arm of saidMach-Zehnder interferometer.
 3. A system as claimed in claim 2, whereinsaid first arm has an interferometry output for coupling light from saidfirst arm and an interferometry input for coupling light into said firstarm.
 4. A system as claimed in claim 3, wherein said light coupled outof said first arm by said interferometry output is reflected by a sampleunder test and coupled into said first arm by said interferometry input.5. A system as claimed in claim 1, wherein said system is an OpticalCoherence Tomography apparatus, said apparatus further comprising: abroadband light source adapted to transmit to said input, a detectoradapted to receive light from said output, and a delay line located ineither said first or said second optical path.
 6. A system as claimed inclaim 1, wherein said system is Wavelength Division Multiplexingapparatus.
 7. A system as claimed in claim 1, wherein said firstwaveguide and said second waveguide have unequal dispersion parameters.8. A system as claimed in claim 1, wherein said dispersion parameter isa second order dispersion coefficient.
 9. A system as claimed in claim1, wherein at least said first waveguide or said second waveguide is anoptical fiber.
 10. A system as claimed in claim 9, wherein at least saidfirst deforming device or said second deforming device is an opticalfiber stretcher.
 11. A system as claimed in claim 1, wherein said firstwaveguide has a first waveguide dispersion modifier coefficient, saidsecond waveguide has a second waveguide dispersion modifier coefficient,and a ratio between said first and second dispersion parameters,multiplied by their respective strain induced waveguide dispersionmodifier coefficients, are unequal.
 12. A method of arranging an opticalsystem, comprising the steps of: adapting an light receiver to receivelight from a light source and couple said input light to at least twooptical paths, arranging a first deformable waveguide in a first opticalpath, said first waveguide having a first dispersion parameter and afirst optical path length, said first waveguide deformable to alter saidfirst dispersion parameter and said first optical path length, arranginga second deformable waveguide in a second optical path, said secondwaveguide having a second dispersion parameter and a second optical pathlength, said second waveguide deformable to alter said second dispersionparameter and said second optical path length, deforming said first andsaid second waveguides adjust said first and second dispersionparameters and maintain said path length of said first optical pathrelative to said second optical path.
 13. A method as claimed in claim12, the method further comprising configuring said first optical path isto be a first arm of a Mach-Zehnder interferometer and configuring saidsecond optical path to be a second arm of said Mach-Zehnderinterferometer.
 14. A method as claimed in claim 13, the method furthercomprising configuring an interferometry output in said first arm hasfor coupling light from said first interferometer arm and aninterferometry input for coupling light into said first interferometerarm.
 15. A method as claimed in claim 13, method further comprisingtransmitting light from said interferometry output to a sample undertest, and receiving light reflected from said sample in saidinterferometry input.
 16. A method as claimed in claim 1, wherein atleast said first deforming device or said second deforming device is anoptical fiber stretcher.
 17. An optical system, comprising: an opticalpath having at least at first and second waveguide, a first waveguidehaving an input to receive light and an output to transmit light, asecond waveguide having an input to receive light and an output totransmit light, said input of said second waveguide configurable toreceive light from said input of said first waveguide, a first deformingdevice adapted to deform said first waveguide, and a second deformingdevice adapted to deform said second waveguide, wherein said firstdeforming device and said second deforming device are operable to adjustsaid dispersion of light in said optical path and said optical pathlength.
 18. A system as claimed in claim 17, wherein said system isWavelength Division Multiplexing apparatus.
 19. A system as claimed inclaim 17, wherein said first waveguide and said second waveguide haveunequal dispersion parameters.
 20. A system as claimed in claim 17,wherein said dispersion parameter is a second order dispersioncoefficient.
 21. A system as claimed in claim 17, wherein at least saidfirst waveguide or said second waveguide is an optical fiber.
 22. Asystem as claimed in claim 17, wherein at least said first deformingdevice or said second deforming device is an optical fiber stretcher.23. A system as claimed in claim 17, wherein said first waveguide has afirst waveguide dispersion modifier coefficient, said second waveguidehas a second waveguide dispersion modifier coefficient, and a ratiobetween said first and second dispersion parameters, multiplied by arespective strain induced waveguide dispersion modifier coefficients,are unequal.
 24. A method of arranging an optical system, comprising:arranging at least a first and second waveguide in series to define anoptical path, said first waveguide having an input to receive light andan output to transmit light, said second waveguide having an input toreceive light and an output to transmit light.
 25. A method as claimedin claim 24, the method further comprising deforming said first and saidsecond waveguides to adjust said dispersion of said light in saidoptical path.
 26. A method as claimed in claim 24, the method furthercomprising transmitting a broadband light source to the input of saidoptical path.